fix: handle parsing variable expression in dockerfile

[ShauryaAg]

fixes: #336

• Add support for parsing variable expressions in Dockerfile as mentioned in Dockerfile references: https://docs.docker.com/engine/reference/builder/#environment-replacement

[ShauryaAg]

@microsoft-github-policy-service agree

[chrmarti]

Thanks for the PR! Left a few comments.

[ShauryaAg]

Known issues:
The current implementation does not support quoted expression variables, that is ${arg:+"word"} will not be evaluated correctly in FROM statements as the fromStatement regex does not parse such FROM lines correctly.

cc: @chrmarti

[chrmarti]

LGTM, thanks! (Getting a second reviewer now before we can merge.)